Symantec has found 31,000 devices with variants of the worm, called Darlloz

A Linux worm that targets routers and set-top boxes is now looking for full-fledged computers to use its new feature, a cryptocurrency mining function, according to Symantec.

Symantec spotted the worm, which it calls Darlloz, in November. It was preloaded with usernames and passwords for routers and set-top boxes that run Linux on Intel's x86 chip architecture and other embedded device architectures such as PPC, MIPS and MIPSEL.

The latest variant of Darlloz, found by Symantec in mid-January, looks for computers running Intel's architecture, wrote Kaoru Hayashi, a senior development manager and threat analyst with Symantec in Japan.

This version of the worm installs "cpuminer," which is an open-source mining program, he wrote. It then begins mining for Mincoins or Dogecoins, two spinoff cryptocurrencies from Bitcoin.

Bitcoins can't be mined efficiently any more by personal computers, but Mincoins and Dogecoins can.

"The reason for this is [that] Mincoin and Dogecoin use the scrypt algorithm, which can still mine successfully on home PCs, whereas Bitcoin requires custom ASIC chips to be profitable," Hayashi wrote.

The attacker using this latest variant of Darlloz mined 42,438 Dogecoins, worth US$46, and 282 Mincoins, worth about $150, by the end of last month.

"These amounts are relatively low for the average cybercrime activity, so we expect the attacker to continue to evolve their threat for increased monetization," Hayashi wrote.

The new Darlloz comes preloaded with 13 login credential combinations, including some for IP cameras, he wrote.

Last month, Symantec scanned the entire address space of the Internet and found 31,716 devices infected with Darlloz in 139 regions, Hayashi wrote. Half of the infections were in China, the U.S., South Korea, Taiwan and India.

More than a third of those infections were on so-called "Internet of things" devices, such as IP cameras, printers, routers and set-top boxes.

The attackers appeared to capitalize on a vulnerability known as a backdoor in several router types, which can be exploited to remotely gain control of the router. But that also posed a threat to Darlloz, as another attacker could boot Darlloz from the system and install another kind of malicious software.

The Darlloz attackers had a solution. "They implemented a feature to block the access to the back door port by creating a new firewall rule on infected devices to ensure that no other attackers can get in through the same back door," Hayashi wrote.

The software on devices such as routers and IP cameras is often not patched as rigorously as other software, such as operating systems, which may give the Darlloz hackers an advantage.

Hayashi wrote that such devices should be patched regularly, have up-to-date firmware and have any default passwords changed. Also, connections to port 23 or port 80 should be blocked if they're not required.